2he Mechanical Action of Badiation. 



9 



But if this transition from a downward to an upward cur- 

 rent is very slow there must be an appreciable time when 

 there is equilibrium, when the air is in contact with a sur- 

 face warmer than itself and yet for a brief period is free 

 from convection currents. 



Again it is well known that the air is perfectly elastic 

 and further that an elastic body will transmit the energy 

 of blows which do not put its mass in motion. Then let 

 us conceive a mass of air, lying between the disk and the 

 surface which receives the heat, subjected on the surface 

 side to a temperature higher than its own and yet free 

 from convection currents. Under these conditions the 

 elastic medium must transmit the heat energy to the disk 

 in a manner not altoo^ether unlike the transmission of the 

 force of a blow by a series of elastic balls. As I conceive 

 the molecular motion it is this. The molecules of air are 

 in motion with a velocity depending upon temperature. 

 When they impinge against the warmed surface they are 

 thrown off with an increased velocity. This velocity is 

 transmitted until the molecules in contact with the disk 

 receive it and they strike the disk with greater energy 

 than do those against the opposite side. This excess of 

 energy drives the disk along. 



If this explanation be correct then while the attraction 

 in air is the manifestation of the well known convec- 

 tion currents, this repulsion in air is the manifestation of 

 a molecular transmission of energy by the air in straight lines 

 outward from a heated surface. lN"ow if such a molecular 

 action do exist then a light body near a heated surface 

 must, in every case, be subject to the influence of these 

 antagonistic forces, being solicited toward the heat by con- 

 vection, and repelled from it by the energy transmitted. 

 Because convection is the more powerful, the motion is 

 toward the source of radiation except when by careful 

 choice of conditions the delicate repulsion can be made 

 visible. 



Trans, ix.'] 2 



